Check when definite (9)
I believe the answer is:
ascertain
'check' is the definition.
('ascertain' can be a synonym of 'check')
'when definite' is the wordplay.
'when' becomes 'as'.
'definite' becomes 'certain' ('certain' can be similar in meaning to 'definite').
'as'+'certain'='ASCERTAIN'
(Other definitions for ascertain that I've seen before include "Establish, find out" , "Find out by making enquiries in sectarian way" , "Become aware" , "Learn" , "settle" .)